The commercial carbon industry manufactures graphite electrodes that are used in electric-arc steelmaking furnaces. These carbon artifacts must carry large electric currents in the steel melting processes. The desirable characteristics of these carbon electrodes are high density, high modulus of elasticity, high electrical conductivity and high flexural strength.
Such electrodes are typically made by mixing petroleum coke with coal-tar pitch having a high solids content including many particles greater than 10 microns, known as binder pitch. The mix is extruded to form a cylinder known as a "green form", which is baked at 900.degree.-1300.degree. C. to volatilize and remove non-carbonaceous material. When the green form is baked, it is transformed from a product which contains about 95% carbon to one which contains greater than 99% carbon. During the baking process, some of the organic compounds are destructively distilled, resulting in carbon deposition in the form. As the vaporized materials vacate their specific locations and exit the form, they produce a porous and channeled structure, resulting in a reduced density and reduced capacity of the form for carrying current. Impregnating pitches are used to fill the pores and channels to increase the carbon density of the form and thus improve the current carrying capacities of the electrode. After impregnation, the form is baked again and then graphitized at temperatures as high as 3000.degree. C.
In the prior art, impregnation required a pitch having a low content of solids greater than about 1 micron in size. Petroleum pitch has been most frequently used in the past because it is relatively free of solid particles; if coal-tar pitch is used, it must have a low solids content to pass the filterability test. Solids content of coal-tar pitch is generally expressed in terms of quinoline insolubles, or "QI", because the particulate matter in coal-tar pitch is largely particles of coal, coke and carbon, which are insoluble in quinoline, while the balance of the pitch is soluble.
More particularly, there are five characteristics normally used to guide the choice of a coal-tar impregnating pitch. These are:
1. Softening point, usually as measured by ASTM D3104. This test gives an indication of pitch viscosity at impregnating conditions. PA1 2. Quinoline Insolubles, (QI), usually is measured by ASTM D2318. This test provides a measure of the coal, coke, and carbon particles in the pitch as well as any liquid crystals that may have formed if the pitch was heat-treated. PA1 3. Ash, usually as measured by ASTM D2415. This test gives an indication of materials that may be left in the electrode that may catalyze carbon loss under ultimate use conditions. PA1 4. Coking value, usually as measured by ASTM D2416. This test gives an indication of how much in-situ carbon will be deposited from the impregnating pitch in the electrode. PA1 5. Rate of filtration and filterability index as measured by any suitable process, which may be similar to that described by Couderc et al in U.S. Pat. No. 4,997,542, column 1, lines 40-65, incorporated herein by reference. Generally, filterability indices of 2.5 g.sup.2 /min. or greater are considered acceptable for an impregnant.
Because of its extremely low solids content and high filterability index, petroleum pitch is normally utilized as the impregnating pitch. However, petroleum pitch has a lower in-situ carbon yield than coal-tar pitch and yields a more non-uniform deposition of its carbon. Coal-tar pitch generally has a lower filterability index because of the coal and coke particles contained therein and is usually subjected to the expensive step of solids removal to make a suitable impregnant.
The present invention enables the economic use of coal-tar pitch as the impregnant for green form electrodes.
As mentioned above, it has been known in the past to use petroleum pitch as an impregnant for carbon electrodes. See U.S. Pat. Nos. 4,961,837 and 4,277,324. These patents of course do not address the problem solved by applicant, which is to prepare a coal-tar pitch economically for such use.
The basic objective of the Couderc et al patent mentioned above (U.S. Pat. No. 4,997,542) is to make a pitch having minimal QI. The present invention has as its object the opposite, in the sense that the quinoline insoluble materials are preserved in the pitch insofar as possible or practical. Couderc et al employ a thermal treatment and flash distillation, and do not centrifuge as does the present invention.
A relatively simple centrifugation of coal tar is shown by Bernet et al in U.S. Pat. No. 4,036,603. While the description says the liquid product is "substantially solid-free" (column 1, line 54), no use is suggested for it, and very likely it would be unsuitable as an impregnation pitch because of residual particles greater than one micron.
Boodman et al, in U.S. Pat. No. 4,436,615, prepare a coal-tar pitch which is proposed for making electrodes. They filter as well as centrifuge, and optionally distill liquids from the separation steps to make a product suggested for impregnating graphite electrodes (column 3, line 68-column 4, line 1).
Mori et al, in U.S. Pat. No. 4,640,761, use a heat-treating step prior to centrifugation to cause aggregation of relatively small particles of quinoline insolubles so they can be more easily removed; in Mori et al U.S. Pat. No. 4,986,895, two centrifugation steps are used with heat treatment between them to cause aggregation of the smaller quinoline insolubles to facilitate centrifugation.
A low QI impregnating pitch is made by Chu et al in U.S. Pat. No. 4,664,774. They use an oxidation system with no resemblance to applicant's.
The only reference of which I am aware utilizing milling actually mills coal-tar pitch rather than coal tar. This is Japanese Patent 63,130,697 (Jun. 2, 1988), which made a pitch capable of impregnating graphite electrodes having a porosity of 17%. The process is not like applicant's, which combines the steps of centrifugation and milling.